What is the main difference between a chemical and a physical change? PLEASE HELP ASAP

Nebulae are enormous clouds in outer space. they are made mostly of hydrogen gas, helium gas, and dust. some nebulae glow brightly, while others do not. the stars that people see are huge, bright balls of glowing gas. they are made mostly of hydrogen and helium. which statement correctly describes other ways in which nebulae and stars are different? a. stars can form inside a nebula but a nebula can never be produced by any star. b. a star always has a higher density than a nebula. c. stars can never form inside a nebula but a nebula can be produced by any star. d. a nebula always has a higher density than a star.

Abaker touches a pie right after taking it out of the oven. which statement best explains why the pie feels hot?

Is it safe to take 450mg of diphenhydramine hydrochloride?